Ross Kemp was left unable to speak after smoking spice as part of a new documentary.

During a visit to HMP Belmarsh as part of his new ITV show about the prison, Ross smoked a “tiny amount of spice residue” that was left in the end of one inmate’s vape.

Writing in The Sun, Ross explained the drug – dubbed Man Down – was made of a “potentially lethal combination of aluminium cleaner and insect killer”, noting: “It does exactly what it says on the tin.”

“I am rarely lost for words but as I turn to camera, I have lost the ability to speak,” he recalled. “My brain has gone blank, my hands are clammy and my mouth is dry.”

Ross revealed that the prisoner whose vape he had used had collapsed after smoking the drug, and had to be revived by guards.

He added: “As he is brought round by staff using citrus — which can knock off some of the effects of Spice — he stares at me through bloodshot, dilated pupils and screams: ‘Get out of my fucking cell.’ When I spoke to him the next day, he could not remember meeting me.

“I asked him why he smoked the drug in his cell, considering he knew it was potentially harmful, and he told me: ‘It takes me out of here for four days.’ This is life in HMP Belmarsh.”

The former-EastEnders-star-turned-documentarian spent six months visiting the famous south-east London prison filming his documentary, which in the past has held Ian Huntley and the killers of Lee Rigby.

Stephen Laxley-Lennon, better known as Tommy Robinson, is also currently incarcerated at HMP Belmarsh, who Ross interviews as part of the documentary.

Welcome To HMP Belmarsh With Ross Kemp airs on Thursday at 9pm on on ITV.